According to U.S. Pat. No. 5,732,888 (JP-A-08-506876), a fuel injection valve includes a needle that axially moves integrally with a movable core by being electromagnetically operated. A coil of the fuel injection valve is supplied with electricity, so that the movable core and a stationary core generate magnetic attractive force therebetween. In this structure, the movable core and the needle integrally move toward the stationary core, so that a contact surface, which is defined on an axial end of the movable core, makes contact with a contact surface, which is defined on an axial end of the stationary core.
When supplying electricity to the coil is terminated, a biasing member such as a spring moves the movable core and the needle integrally toward a valve seat on the opposite side of the stationary core, so that the contact surfaces are separated from each other.
The contact surfaces of the cores cause a squeeze effect therebetween, immediately after terminating electricity supplied to the coil so that the contact surfaces of the cores start separating from each other. The squeeze effect disturbs movement of the movable core. In this condition, fuel hard to flow into the gap between the contact surfaces. As a result, response of the fuel injection valve becomes low with respect to the termination of electricity. Consequently, a period, which is between terminating electricity to the coil and starting fuel spray from the fuel injection valve, becomes long.
In the structure of U.S. '888, the fuel injection valve includes cores each having an end partially defining a protrusion. In this structure, the force caused by squeeze effect can be reduced, compared with a structure in which each end of both cores entirely defines a substantially flat contact surface. However, when the protrusion is simply defined in the structure of U.S. '888, magnetic flux for generating attractive force is concentrated to the protrusion defined in the axial end. The contact surface defined in the protrusion is less than the entire surface of the axial end of the core. Accordingly, magnetic flux concentrated to the protrusion is saturated at low level. Furthermore, in U.S. '888, each axial end of both of the cores has a non-contact surface, in which the protrusion is not defined. The non-contact surface defines a gap when the cores make contact with each other, and consequently, magnetic flux is reduced between the cores. Accordingly, magnetic attractive force decreases due to reduction in magnetic flux in the axial end of the core, compared with the structure, in which the axial ends entirely define contact surfaces. As a result, response of the fuel injection valve becomes low when the coil is supplied with electricity.